Analysis of chemical structure of wood charcoal by X-ray photoelectron spectroscopy

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J Wood Sci (1998) 44:56-61 © The Japan Wood Research Society 1998

Koei Nishimiya • Toshimitsu Hata • Yuji Imamura Shigehisa Ishihara

Analysis of chemical structure of wood charcoal by X-ray photoelectron

spectroscopy*

Received: April 22, 1997 / Accepted: August 18, 1997

Abstract W o o d charcoal carbonized at various tempera-

tures was analyzed by X-ray photoelectron spectroscopy (XPS), Fourier-transform infrared spectroscopy (FTIR), and X-ray diffractometry to investigate the changes of chemical structures during the carbonization process. From the infrared spectra, the carbon double bonds and aromatic rings were seen to form at a carbonization temperature of about 600°C. F r o m the XPS spectra, the ratio of aromatic carbons increased in the temperature range 800-1000°C and over 1800°C. The condensation of aromatic rings pro- ceeded as carbonization progressed. The drastic reduction of electrical resistivity of charcoals was observed in almost the same temperature range. It was found that the conden- sation of aromatic rings had some relation to the decline in electrical resistivity. W o o d charcoal carbonized at 1800°C was partly graphitized, a finding supported by the results of X-ray diffraction and XPS. The functional groups contain- ing oxygen diminished with the increase in carbonization temperature.

Key words W o o d charcoal • Chemical structure • X-ray

photoelectron spectroscopy • X-ray • Carbonization

Introduction

Much attention has been paid recently to the use of wood charcoal as a source of carbon materials. It has excellent characteristics; moreover, it can be replaced, as we can uti-

K. Nishimiya ([~) • T. Hata - Y. Imamura - S. Ishihara

Wood Research Institute, Kyoto University, Uji, Kyoto 611, Japan Tel. +81-774-38-3677; Fax +81-774-38-3678

e-mail: kouei@kuwri.kyoto-u.ac.jp

* This paper was presented at the 45th Annual Meeting of the Japan Wood Research Society in Tokyo, April 1995 and at the 47th Annual Meeting of the Japan Wood Research Society in Kochi, April 1997

lize sustainable forest resources. Highly fire-endurant or electromagnetically shielding materials 1-4 were developed from wood charcoal with novel performances equivalent to those of metallic materials. They can be used as adsorbents for environmental purification and as materials regulating humidity. 5-7

The carbonization process of wood has been the focus for the development of new carbon materials. W o o d under- goes several stages of conversion during the carbonization process, 8 and the chemical structures and properties of wood charcoal are assumed to change depending on the carbonization temperature. The characteristics of wood charcoal are essentially related to the chemical structures formed during the heating process.

Until now, little knowledge has been obtained about the relation between the chemical structure of charcoal and the carbonization temperature. For example, the electrical re- sistivity of charcoal varies from the insulator to the conduc- tor regions depending on the carbonization temperatures4'9; as a result, the charcoal develops an electromagnetic shield- ing property. The electrical conductivity of charcoal is be- lieved to originate from its chemical structure, which is still unknown. The changes in the chemical structure of w o o d charcoal were shown to have some effect on its adsorption properties, which are influenced by the carbonization tem- perature. To design new w o o d carbon composites with high and multiple functions, it is necessary to clarify the changes in the chemical structure of wood charcoal during the car- bonization process.

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Sugi

(Cryptomeria japonica

D. Don) logs about 25 years old were obtained from Kochi Prefecture and milled into powder (20 mesh pass), Commercial cellulose powder (No. MN-100; Nacalai Tesque, Japan) was used as a control. The wood meal and cellulose powder were oven-dried at 105°C for 24h before testing.

Carbonization

For carbonization temperatures below 1400°C, the wood meal and cellulose powder were placed in a heat-resistant box that was covered with layers coke and heated in an electric oven, increasing the temperature 4°C/min up to the desired temperature in an inert atmosphere under normal pressure. After the target temperature was attained, it was maintained for 3 h; it was then allowed to cool naturally, and the charcoal was removed from the oven.

For carbonization temperatures above 1500°C, the samples were precarbonized at 700°C in a nitrogen gas at- mosphere to remove the volatile substances and tar in- cluded in the raw materials. The precarbonized charcoal was placed in a carbon crucible and then in an ultra-high temperature electric oven (SHC-3000H; ShinMaywa, Japan) and carbonized in an argon gas atmosphere. The specimens were heated at a rate of 10°C/min and maintained for 1 h at the target temperature. Finally, the furnace was cooled to 600°C at the same rate as the heating process to protect the carbon electrode and then cooled naturally.

Charcoal characterization

The FTIR, XPS, and X-ray diffraction methods were used to investigate the changes in the functional groups and the chemical structures in the wood charcoal. The wood char- coal and original materials were dried at 105°C for 24h before analysis.

Fourier-transform infrared spectroscopy

The infrared spectra of the wood charcoal and carbonized cellulose were obtained with a F T I R apparatus (FT-IR 7000; Nihon Banko, Japan) using the KBr pellet technique. Charcoal powder of 1-2rag were mixed with 200rag KBr, and a pellet was formed by pressing.

X-ray diffraction

chamber was adjusted to under 10 8mmHg. The spectra were referenced by setting 285.0eV for the Cls aliphatic carbon peak to eliminate the charge effect in the case of wood powder and charcoal carbonized at lower temperatures. The surfaces of the charcoals were etched for 30 s by argon gas before the XPS analysis to exclude the effects of im- purities present on the surfaces of the samples. The survey spectra of wood charcoal and cellulose Cls and Ols, respec- tively were obtained and subjected to background elimina- tion and peak analysis. For peak analysis, the decomposed peak positions were assigned according to the recent litera- ture. .1'12 The assignments of Cls peak components are

shown in Table 1. The peak analysis was performed by means of the peak decomposition to fit a gaussian function by the data-processing program VISION supplied by Shimadzu and KRATOS.

Results and discussion

Change in chemical composites in charcoal investigated by FTIR

The FTIR spectra of the original cellulose and those car- bonized at 300, 600, and 800°C are shown in Fig. 1. The infrared spectrum of the charcoal carbonized at 800°C was not as clearly resolved because of the larger absorbance of visible rays of light by charcoal due to its black color. The same tendency was found in the charcoal carbonized at more than 800°C. The spectrum of petroleum pitch be- comes obscure because of the increased electrical conduc- tivity of the samples) ° A carbonization temperature of 800°C is thought to be the limit for the measurement of FTIR spectra of charcoal by the KBr pellet technique.

At first, the O - H stretching vibrational mode at about 3300cm < decreased with increasing carbonization tem- perature and mostly diminished at a carbonization tempera- ture of 800°C compared to the original material. The C = O mode at about 1700cm -1 clearly appeared at 300°C. The C = C mode near 1660-1630cm -1 was weakly detected in the spectra of the charcoal carbonized at 300-600°C. The aromatic mode at about 1600cm < was weak at 600°C.

Based on these results, a decrease in the O - H mode after increasing the heating temperature indicated dehydration

The X-ray diffractograms were obtained with a XRD appara- tus (RINT 1400; Rigaku Denki, Japan) using Cu-K~. radia- tion. The measurement conditions were as follows: 50 kV tube voltage, 80mA tube current, and 4°/min scanning speed.

X-ray photoelectron spectroscopy

The XPS spectra of charcoal and original materials were obtained on a XPS apparatus (AXIS-HS-1; Shimadzu/

Table 1. Positions and assignments of Cls peak components

Name Binding type Binding energy (eV)

C1 Graphite/aromatic 284.6

C2 Aliphatic 285.0

C3 C-OH,C-O-C 286.1

C4 C=O 287.6

C5 COOH 290.1

C6 ~ - ~* 291.2

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58

I I I I I I

Wave number (cm -I)

Fig. 1. Fourien-transform infrared spectroscopy (FTIR) spectra of car- bonized cellulose

Numbers in the figure indicate carbonization temperatures

///f•105

1.5 105o1¢/

O . . . / 300

3O0o /

0 5 400 .."///

• 600 .X~...~400 • cellulose

.... o sugi wood

. ~ ~N,600

I I I

0 0.25 0.5 0.75

O/C

ratio

Fig. 3. Krevelen diagram for carbonized materials. Carbonization tem- peratures are shown near the plots in this diagram

ID e " t ~ , . Q O ra~

I I

4000 3000 2000 1500 1000 500

Wave

n u m b e r ( c m -1) Fig. 2. FTIR spectra of wood charcoal

of the charcoal, which was proved by the formation of a C = O peak that appeared on the charcoal heated at 300°C, but disappeared at 600°C. This sequence suggested that double bonds were formed in the charcoal by dehydrogena- tion during the carbonization process. These results are in accordance with the carbonization behavior of cellulose de- scribed previouslyJ 3

The infrared spectra of original and carbonized sugi wood are shown in Fig. 2. In the case of the original wood, aromatic absorption originating from lignin TM was detected

at about 1600 and 1510cm -1. These aromatic modes were also found in the charcoal but moved to a lower wave num-

ber with the increase in carbonization temperature. This result suggested that the aromatic mode due to lignin was changed to a different type of aromatic compound by the carbonization. The aromatic outer-vibration mode ap- peared at about 900-685 cm 1 in the charcoal samples after carbonization.

The C = O mode was detected at about 1740cm -1 in original wood due to hemicellulose, although the mode at about 1700cm i showed maximum absorbance at a carbon- ization temperature of 300°C. The C = O mode seemed to be formed by decomposition of cellulose, as shown in Fig. 1. The O--H mode at 3300cm -1 decreased with an increasing carbonization temperature.

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1400 C o

0~ ~ 1000 C

8 0 0 ° C

600°(2

300°C -~

,original I ~

20 40 60 80 ~=

2 0 ( °

)

Fig. 4. X-ray diffractograms of wood charcoal

C 1 s peak

__ 2400°C., ^Ols peak . .-.-..-~

1 800°C 1

. - . _c . . . . k- - -

14°°°c

0 o

I , I , I , I

700 600 500 400 300

B i n d i n g e n e r g y ( e V )

2OO

Fig. 5. X-ray photoelectron spectroscopy (XPS) survey spectra of wood charcoal carbonized at various temperatures

X-ray diffraction

X-ray diffractograms of wood charcoal are shown in Fig. 4. Peaks originating from cellulose were detected from original wood. These peaks disappeared when the wood was car- bonized even at 300°C, a result that indicated decomposi- tion of the crystal structure of cellulose by thermal decomposition of cellulose. No clear diffraction peaks were detected at temperatures below 1400°C, suggesting no graphitization in the charcoal. The X-ray diffraction pattern changed gradually with the increase in temperature, possi- bly owing to ultrastructural changes in the charcoal. The sharp peaks that seemed to originate from the graphite crystalline structure appeared in charcoal carbonized at 1800°C and 2000°C.

1

//////\

, , , ,/ ,

300 295 290 285 280

B i n d i n g e n e r g y ( e V )

Fig. 6. Cls spectra of wood charcoals carbonized at various tempera- tures and graphite. The graph at upper left is the magnification around the binding energy 288-294eV

Charcoal investigated by X-ray photoelection spectroscopy

The XPS survey spectra of wood charcoal produced from sugi carbonized at various temperatures are depicted in Fig. 5 at a binding energy of 200-700 eV. The Cls (about 285 eV) and Ols (about 532eV) photoelectron peaks were clearly resolved. Photoelectron peaks originating from other atoms were small, which shows that the charcoal consists of mainly carbon and oxygen. The intensity of Ols peak decreased with higher carbonization temperatures, so the carbon at- oms increased against the oxygen atoms.

The Cls spectra were chosen to investigate the chemical structure of charcoal. XPS is sensitive to the state of the samples. A few charcoal samples were influenced by surface oxidation when there was no etching, so etching was under- taken. Because the spectra seldom changed while increasing the etching time, the inner structure of charcoal is repre- sented on the surface after etching. As shown in Fig. 6, the peak width of the Cls spectra of the charcoal carbonized at lower temperatures was broad, and the maximum peak po- sition shifted at a higher binding energy. The Cls spectra tended to shift at the higher binding energy when the car- bon atoms were combined with oxygen atoms.

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60

t~

e~

C1

290 285

Binding energy (eV)

I

295 280

Fig. 7. Example of peak extraction of Cls peak of charcoal (refer to Table 1 )

T a b l e 2. Area of Cls peaks of wood charcoal Carbonization

temperature(°C)

Area of the Cls peak (%)

c1 C2 C3 C4 c5 C6

. - 6 "N o

4

1~(?1

I

[

I

I

0 500 1000 1500 2000 2500

Carbonized temperature (°C)

Fig..8. Relation between carbonizing temperature and the Ca,o/C,j ~ ratio obtained by XPS

Original 23.40 3 9 . 8 6 2 1 . 4 2 12.59 2.69 0.06 300 11.89 4 8 . 9 0 24.31 9.93 3.08 1.74 400 8.82 5 3 . 6 9 2 1 . 0 2 10.09 4.86 1.51 600 19.71 3 8 . 5 6 2 2 . 8 0 12.35 4.38 2.20 700 12.38 6 0 . 0 7 16.68 7.66 3.04 0.14 800 31.00 3 3 . 9 5 1 5 . 8 2 13.01 4.03 2.19 1000 44.66 2 4 . 0 4 15.75 9.24 3.68 2.65 1400 44.94 2 2 . 5 5 16.87 9.61 3.83 2.21 1800 44.21 2 5 . 3 1 16.31 8.52 3.80 1.85 2000 55.06 1 1 . 2 8 18.44 9.29 3.88 2.05

2400 64.77 9.93 15.75 5.39 1.97 2.19

For explanation of C1-C6 refer to Table 1.

P e a k extraction

To e v a l u a t e the chemical structures of the surface of char- coal, p e a k e x t r a c t i o n was p r o c e s s e d to the C l s spectra. A l l of the C l s s p e c t r a were c o n s i d e r e d to consist of c o m p o n e n t s r e l a t e d to c a r b o n - c o n t a i n i n g functional groups. The chemi- cal binding state in charcoal can be distinguished b y analyz- ing the c o m p o n e n t s e x t r a c t e d f r o m the C l s spectra. T h e e x t r a c t e d p e a k s a p p e a r e d as in Fig. 7 by curve fitting. In this case the p e a k s c o r r e s p o n d e d to those of a r o m a t i c carbon, aliphatic carbon, the C = O p e a k , or the C - O H peak. T h e areas of the C l s p e a k c o m p o n e n t s w e r e calculated and are shown in T a b l e 2.

T h e r e l a t i o n b e t w e e n the c a r b o n i z e d t e m p e r a t u r e and the a r o m a t i c c a r b o n s / a l i p h a t i c carbons ratio

(Caro/Cali)

is r e p r e s e n t e d in Fig. 8. This ratio is t h o u g h t to b e a reflection of the a m o u n t of a r o m a t i c or c o n d e n s e d a r o m a t i c carbons in the charcoal.

T h e Caro/C~ ratio was less than 0.5 at a carbonizing tem- p e r a t u r e < 7 0 0 ° C and r o s e to a b o u t 2.0 at a r o u n d 1000°C. This value r e m a i n e d almost constant until 1800°C and then

rose again to 5.0 at 2000°C. T h e

Caro/Cal i

ratio of g r a p h i t e was a b o u t 20, which was m u c h higher t h a n that for w o o d charcoal. T h e rise in the Caro/C~i ratio at 800-1000°C is due to f o r m a t i o n of s o m e c o n d e n s e d a r o m a t i c rings, which m a y n o t i c e a b l y influence the electrical resistance of the charcoal. F u k u d a et al. 2~ suggested that the electrical conductivity of c a r b o n m a t e r i a l s is due to c o n d e n s e d a r o m a t i c carbons and a few o x y g e n atoms. Charcoals f r o m various types of w o o d c a r b o n i z e d at these t e m p e r a t u r e s have similar chemical structures, so the electrical conductivity of the charcoals is p r o b a b l y the same. In this case, the oxygen atoms m a y act as a d o r p a n t , z2 O n the o t h e r hand, the rise in the Caro/Ca~ ratio at 1800°C was possibly due to the growth in the g r a p h i t e crystalline structure, which was confirmed b y X - r a y diffrac- tion of the charcoal.

The c o m p o n e n t s of C 3 ( C - O H ) , C 4 ( C = O ) , and C 5 ( C O O H ) , which c o n t a i n e d oxygen atoms, g r a d u a l l y de- c r e a s e d with an increasing c a r b o n i z a t i o n t e m p e r a t u r e . T h e p e a k s for C3 and C4 were d e t e c t e d for almost all carboniza- tion t e m p e r a t u r e s . A p p a r e n t l y , the functional groups in the charcoal d e c r e a s e d with the c a r b o n i z a t i o n t e m p e r a t u r e , and the p r o p o r t i o n of the O H g r o u p b e c a m e c o m p r e h e n - sively higher than those of o t h e r functional groups as a whole. It was due to the fact that the O l s p e a k of the charcoal c a r b o n i z e d at higher t e m p e r a t u r e s was small, even t h o u g h the functional groups of hydroxyl and carbonyl s e e m e d to b e p r e s e n t in the charcoal c a r b o n i z e d at the higher t e m p e r a t u r e s .

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w o o d was n o t d e t e c t e d by XPS.

In the case of cellulose, the u n e x p e c t e d p e a k due to c a r b o n y l g r o u p s was d e t e c t e d in the XPS s p e c t r u m of origi- nal cellulose b y curve fitting. T h e peak, which was not as strong, might be due to the effect of etching. It m a y also be due to the limitation of the a l g o r i t h m for curve fitting, which leads to a n o n e x i s t e n t p e a k w h e n p e a k tailing is distin- guished. T h e p e a k of c a r b o n y l g r o u p was d e t e c t e d in cellulose c a r b o n i z e d at 300°C, b u t it d e c r e a s e d at 600°C, in a c c o r d a n c e with the results from F T I R . T h e changes in functional groups of charcoal w e r e a s s u m e d to b e basi- cally d e t e r m i n e d b y m e a n s of the p e a k e x t r a c t i o n using XPS.

Conclusions

T h e c a r b o n i z a t i o n of w o o d at t e m p e r a t u r e s b e l o w 800°C p r o c e e d s as follows. T h e crystalline structure of cellulose was d e s t r o y e d , a n d c a r b o n y l groups w e r e g e n e r a t e d at a b o u t 300°C, which s e e m e d to be the early stage of c a r b o n - ization. T h e d i s a p p e a r a n c e of c a r b o n y l groups and the gen- e r a t i o n of c a r b o n d o u b l e b o n d s and a r o m a t i c rings w e r e observed. D e h y d r a t i o n a n d an increase in the p r o p o r t i o n of c a r b o n o c c u r r e d > 6 0 0 ° C with a g r a d u a l increase in the c o n d e n s a t i o n of a r o m a t i c rings. It was also shown by XPS analysis that the functional groups containing o x y g e n de- c r e a s e d d u r i n g the c a r b o n i z a t i o n process. Basically, the re- sults o b t a i n e d b y F T I R w e r e a c c o r d a n c e with those of XPS using the curve-fitting technique.

X - r a y p h o t o e l e c t r o n s p e c t r o s c o p y and X - r a y diffrac- tion analysis w e r e u s e d to e v a l u a t e the chemical struc- ture of w o o d charcoal c a r b o n i z e d at high t e m p e r a t u r e s b e c a u s e the F T 1 R spectra were n o t clear owing to s o m e limitations, d e s c r i b e d above. It was r e v e a l e d by XPS that the p r o p o r t i o n of a r o m a t i c c a r b o n rose at t e m p e r a t u r e s of 800-1000°C and 1800°C. T h e X - r a y diffractograms s h o w e d sharp diffraction p e a k s a b o v e 1800°C. It was b e l i e v e d t h a t c o n d e n s a t i o n of the a r o m a t i c rings b e g a n at a b o u t 800°C, and the structure of the ~ - e l e c t r o n sys- t e m was g e n e r a t e d even t h o u g h it did not attain perfect graphitization. A b o v e 1800°C, the c o n d e n s e d a r o m a t i c rings in charcoal c h a n g e d to g r a p h i t e in terms of structure. A drastic r e d u c t i o n in the electrical resistivity of charcoal was o b s e r v e d in the s a m p l e c a r b o n i z e d at a b o u t 800°C in a c c o r d a n c e with the change in chemical structure of w o o d charcoal. It was a s s u m e d to b e the r e a s o n the ~ - e l e c t r o n system was g e n e r a t e d by the c o n d e n s a t i o n of a r o m a t i c rings during c a r b o n i z a t i o n . It was also suggested that the electrical conductivity of w o o d charcoal at r o o m t e m p e r a - ture o r i g i n a t e d m a i n l y f r o m g e n e r a t i o n of the re-electron system.

Ministry of Education, Science and Culture, Japan.

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Figure

Table 1. Positions and assignments of Cls peak components

Table 1.

Positions and assignments of Cls peak components p.2
Fig. 1. Fourien-transform infrared spectroscopy (FTIR) spectra of car- Numbers bonized cellulose in the figure indicate carbonization temperatures
Fig. 1. Fourien-transform infrared spectroscopy (FTIR) spectra of car- Numbers bonized cellulose in the figure indicate carbonization temperatures p.3
Fig. 3. Krevelen diagram for carbonized materials. Carbonization tem- peratures are shown near the plots in this diagram
Fig. 3. Krevelen diagram for carbonized materials. Carbonization tem- peratures are shown near the plots in this diagram p.3
Fig. 2. FTIR spectra of wood charcoal
Fig. 2. FTIR spectra of wood charcoal p.3
Fig. 6. Cls spectra of wood charcoals carbonized at various tempera- tures and graphite
Fig. 6. Cls spectra of wood charcoals carbonized at various tempera- tures and graphite p.4
Fig. 5. X-ray photoelectron spectroscopy (XPS) survey spectra of wood charcoal carbonized at various temperatures
Fig. 5. X-ray photoelectron spectroscopy (XPS) survey spectra of wood charcoal carbonized at various temperatures p.4
Fig. 4. X-ray diffractograms of wood charcoal
Fig. 4. X-ray diffractograms of wood charcoal p.4
Table 2. Area of Cls peaks of wood charcoal

Table 2.

Area of Cls peaks of wood charcoal p.5
Fig. 7. Example of peak extraction of Cls peak of charcoal (refer to Table 1 )
Fig. 7. Example of peak extraction of Cls peak of charcoal (refer to Table 1 ) p.5
Fig..8. Relation between carbonizing temperature and the Ca,o/C,j ~ ratio obtained by XPS
Fig..8. Relation between carbonizing temperature and the Ca,o/C,j ~ ratio obtained by XPS p.5

References

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